Developing

Getting started

A snap is a bundle of your app and its dependencies that works without modification across many different Linux distributions. Snaps are discoverable and installable from the Snap Store, an app store with an audience of millions.

The following sections provide a basic overview of using snaps, from their installation to their management and manipulation.

Installing snapd

The snapd daemon manages the snap environment on the local system. Its installation will include the snap tool for interacting with snaps. You can check both are installed by attempting to run snap version on the command line:

If all is well, version numbers for the main system components will be output, as shown above, albeit with potentially different versions and Linux distribution identifier.

If the snap command is not found, or if the command takes a while to respond before reporting that snapd is unavailable, make sure snapd is properly installed before continuing.

The "$" above represents the command line prompt. Sample commands come after this prompt with any relevant output shown below.

Searching for snaps

With snapd installed, snaps can be discovered, searched for, and installed from the Snap Store, from the Snap Store desktop app, and directly from the command line. For example, the following command returns a list of media players:

The ✓ alongside videolan in the above output indicates that the snap publisher has been verified.

Learning about a snap

The snap info command makes it easy to find more details about a specific snap. These details include what a snap does, who publishes it, the command(s) it provides and which channel versions are available for installation:

Installing a snap

Installing a snap is straightforward:

$ sudo snap install vlc

Channels are an important snap concept. They define which release of a snap is installed and tracked for updates. The stable channel is used by default, but opting to install from a different channel is easily accomplished:

$ sudo snap install --channel=edge vlc

After installation, the channel being tracked can be changed with:

$ sudo snap switch --channel=stable vlc

The sudo command ensures the command following it is executed as the root administrative user.

Running snap commands

A snap’s installed applications can be found under /snap/bin, and subsequently, often added to $PATH. This makes commands directly accessible from the command line.

For example, the command installed via the VLC snap is simply vlc:

$ which vlc
/snap/bin/vlc

If executing a command directly doesn’t work, try prefixing it with the /snap/bin path:

Some snaps, such as core listed above, are installed by automatically by snapd to satisfy the requirements of other snaps.

Refreshing an installed snap

Snaps are updated automatically. However, to manually check for updates, use the following command:

$ sudo snap refresh vlc

The above will check the channel being tracked by the snap. If a newer version of the snap is available, it will be downloaded and installed.

Changing the channel being tracked and refreshing the snap can be accomplished with a single command:

$ sudo snap refresh --channel=beta vlc

Updates are automatically installed within 6 hours of a revision being made to a tracked channel, keeping most systems up-to-date. This schedule can be tuned via configuration options.

Versions and revisions

Versions and revisions convey different details about one specific release of a snap:

Version : the version of the software being packaged, as assigned by the developers

Revision: the sequence number assigned by the store when the snap file was uploaded

The version is a string that was assigned to a project by its developers, according to their development practices. It tells the user what content to expect from a snap. The revision is an automatic number assigned by the Snap store, giving the snap it a unique identity within the channel.

Neither the version nor the revision enforce an order of release. The local system will simply attempt to install whatever snap is recommended by the publisher in the channel being tracked.

Reverting an installed snap

A snap may be reverted to a previously used revision with the snap revert command:

$ sudo snap revert vlc
vlc reverted to 3.0.5-1

This operation will revert both the snap revision and the data associated with the software. If the previously used revision of the snap is from a different channel, that snap will be installed but the channel being tracked won’t change.

A snap won’t automatically update to a version previously reverted from, and the output from snap refresh will continue to state All snaps up to date. A reverted snap will be automatically updated when a new and different revision is made available by the publisher.

However, explicitly adding the snap name to snap refreshwill update the snap, regardless of whether the latest revision was previously reverted from or not:

On an Ubuntu Core system, such as Ubuntu Core 18 and Ubuntu Core 16, reverting twice will work too. This is because snapd attempts to keep three revisions of a snap: the most recently installed plus the two previous installations. On classic systems like Ubuntu 18.04 LTS or Ubuntu 16.04 LTS, two revisions are retained by default. This behaviour can be modified with the refresh.retain system option.

Listing all available revisions

The following command lists all revisions available for every installed snap, and also highlights which particular revisions are disabled at the moment:

Removing a snap

To remove a snap from your system, along with its internal user, system and configuration data, use the remove command:

$ sudo snap remove vlc
vlc removed

By default, all of a snap’s revisions are also removed. To remove a specific revision, add the --revision=<revision-number> argument to the remove command.

Prior to removal (except on Ubuntu Core systems), a snap’s internal user, system, and configuration data is saved as a snapshot (snapd 2.39+), and retained for 31 days. A snapshot can be used to restore the state of your snap upon reinstallation. See Snapshots for further details.